Multiple computing devices can access content via multiple computer network connections on one or more different computer networks at different times. Online content may be received at multiple computing devices from various first-party or third-party sources. First-party content can include the primary online content requested or displayed by a user's device. For example, first-party content may be a webpage requested by the client or a stand-alone application running on a computing device. Third-party content can include additional content that may be provided in conjunction with the first-party content. A computer system that accesses computer network data may be unable to efficiently determine related computer network activity between multiple different computing devices that can occur during multiple, different computer network sessions established with the respective computing devices.